It is known to crosslink polymers by means of additives. Crosslinking improves properties of the polymer such as mechanical strength and heat resistance. Polymers normally considered to be thermoplastics, and not crosslinkable, can also be made crosslinkable by introducing crosslinkable groups in the polymer. Examples thereof are polymer compositions comprising polyolefins, such as polyethylenes, where silane compounds have been introduced as crosslinkable groups, e.g. by grafting silane compounds onto a prepared polyolefin, or by copolymerisation of an olefin and a silane compound. Such techniques are known e.g. from U.S. Pat. No. 4,413,066, U.S. Pat. No. 4,297,310, U.S. Pat. No. 4,351,876, U.S. Pat. No. 4,397,981, U.S. Pat. No. 4,446,283 and U.S. Pat. No. 4,456,704.
The crosslinking of polymer compositions comprising hydrolysable silane groups with catalysts is known in the art, see e.g. EP0736065. It is further known that the crosslinking process may advantageously be carried out in the presence of acidic silanol condensation catalysts. The acidic silanol condensation catalysts permit crosslinking of silane-containing polymer compositions already at room temperature (about 20 to 25° C.). Examples of such acidic silanol condensation catalysts which are organic sulphonic acids, or precursors of such acids, are disclosed in, for example, WO95/17463, EP1309631, EP1309632 and EP1849816, which documents, and the contents therein, are enclosed herein by reference.
It is also known to use various additives in polymer compositions. The particular type and amount of the additive to be used is dependent on the particular application a polymer composition is designed for.
Further, some important groups of additives are surfactant interacting additives which may be represented by, for example, pigments, and important pigments are pigments which comprise titanium dioxide, i.e. TiO2, and/or calcium carbonate, i.e. CaCO3, as the main component. The pigments which comprise titanium dioxide, and/or calcium carbonate, are used to improve colour coverage of processed articles. The pigments, comprising titanium dioxide, and/or calcium carbonate, are, e.g., used to make processed articles more opaque.
Furthermore, uses of pigments comprising titanium dioxide, and/or calcium carbonate, in polymer compositions comprising polyolefins with hydrolysable silane groups and conventional silanol condensation catalysts are also known in the art. Moreover, it has further been shown that surfactant interacting additives, such as pigments comprising titanium dioxide, and/or calcium carbonate, are to some extent deactivating the crosslinking ability of silanol condensation catalysts of the Brönstedt acid type. Therefore, generally speaking, pigments comprising titanium dioxide, and/or calcium carbonate, are not compatible with the technology of crosslinking polyolefins containing hydrolysable silane groups with the silanol condensation catalysts of the Brönstedt acid type. Thus, there is a need of polymer compositions comprising a specific silanol condensation catalyst and a surfactant interacting additive, for example pigments comprising titanium dioxide, and/or calcium carbonate, wherein the polymer compositions effectively promote the crosslinking performance of the silanol condensation catalysts of the Brönstedt acid type.